In this multidisciplinary project, researchers from Biomedical Engineering, Pediatric Cardiology, Physics, and Mathematics will combine theoretical and experimental approaches to investigate the stability of cardiac response under rapid pacing. As the pacing rate increases, cardiac muscle exhibits beat-to-beat changes in the action potential duration (APD alternans). APD alternans and its clinical manifestation, T-wave alternans, are associated with increased vulnerability for arrhythmias. Thus, increased understanding of the rhythm stability coming from the proposed research will lead to the development of new techniques for detection of the precursors of arrhythmias and for identifying patients at risk for fibrillation and tachycardias. Specific Aims are: (1) Develop a new, more robust experimental protocol for determining stability of the cardiac response pattern. (2) Investigate whether steady-state alternans occurring in spatially extended, homogeneous tissue is always discordant (i.e., APD oscillations in some regions of the tissue are out of phase with the oscillations at the pacing site). (3) Construct an optical fiber-based transmural mapping system that allows mapping action potentials in three dimensions. The research will start with mathematical analysis that uses idealized models of membrane kinetics. Analytical results will be tested through computer simulations, first involving idealized membrane models under space clamp conditions, then progressing to physiologically accurate membrane models in up to three spatial dimensions. Some models will also take into account transmural APD heterogeneity. Concurrently, analytical results will be tested experimentally. Initial tests will use in-vitro preparations of bullfrog ventricle, which are relatively, and progress to in-vitro wedge preparations of rabbit ventricle, which exhibit transmural APD heterogeneity and anisotropy typical for mammalian hearts. The three-way comparisons between results from theory, computer simulations, and experimental studies will allow us to refine and, if needed, expand the mathematical theory and computer models, with each iteration leading to more complete understanding of cardiac dynamics.